As per NACE MR0175/ISO 15156, both grades are acceptable for any partial pressure of H2S. The only restriction (as per Part 2, Table A.3) is that N80 can only be used at temperatures less than 225°F. L80 can be used at all temperatures. Note that L80 refers to L80 Type 1. Also, as with other...
API Type 6BX Flanges (as defined in API 6A) that use BX gaskets are designed to be made up face-to-face with no gap between the flange faces. After make-up there is often a small gap though depending on machining tolerances and make-up torque applied.
